A typical LCD device includes a multiplicity of pixel units and liquid crystal molecules at the pixel units. The LCD device utilizes the liquid crystal molecules to control light transmissivity of each pixel unit. The liquid crystal molecules are driven according to external video signals received by the LCD device. A conventional LCD device generally employs an inversion driving method to drive the liquid crystal molecules, in order to protect the liquid crystal molecules from decay or damage.
Referring to FIG. 7, a schematic, exploded, side cross-sectional view of a conventional LCD device is shown. The LCD device 10 includes a first substrate 11, a common electrode 12, a first alignment film 13, a liquid crystal layer 14, a second alignment film 15, a plurality of pixel electrodes 16, and a second substrate 17. The first substrate 11 is parallel to the second substrate 17. The common electrode 12 is disposed on an inner surface of the first substrate 11. The plurality of pixel electrodes 16 are disposed on an inner surface of the second substrate 17, and are arranged in a matrix. The first alignment film 13 is coated on the common electrode 12, and the second alignment film 15 is coated on the plurality of pixel electrodes 16. The liquid crystal layer 14 is sandwiched between the first alignment film 13 and the second alignment film 15. Each of the pixel electrodes 16, part of the common electrode 12 generally opposite to the pixel electrode 16, and liquid crystal molecules (not labeled) sandwiched therebetween cooperatively define a pixel unit (not labeled).
Data voltages generated by a data driving circuit (not shown) are provided to the plurality of pixel electrodes 16, and a common voltage generated by a common voltage generating circuit (not shown) is provided to the common electrode 12. In each pixel unit, an electric field is generated between the pixel electrode 16 and the common electrode 12. The electric field controls rotating angles of the liquid crystal molecules of the pixel unit, and the rotating angles determine the light transmissivity of the pixel unit. The light transmissivity of each pixel unit determines a brightness of the pixel unit. The LCD device 10 displays images by controlling the brightness of each of the pixel units.
Referring also to FIG. 8, a waveform diagram of the data voltage and the common voltage applied to one of the pixel units is shown. In frame n−1, a value of the data voltage is Vdata1, and a value of the common voltage is Vcom1, where Vdata1<0, Vcom1<0, and Vdata1>Vcom1. A value of the electric field of the pixel unit is (Vdata1−Vcom1)/d, where d is a vertical distance between the common electrode 12 and the pixel electrode 16. A direction of the electric field of the pixel unit is from the pixel electrode 16 to the common electrode 12. In frame n, the value of the data voltage is Vdata2, and the value of the common voltage is Vcom2, where Vdata2>0, Vcom2>0, Vdata2<Vcom2, Vcom2=−Vcom1, and Vcom2−Vdata2=Vdata1−Vcom1. The value of the electric field of the pixel unit is (Vdata2−Vcom2)/d. The direction of the electric field of the pixel unit is from the common electrode 12 to the pixel electrode 16. In frame n+1, the value of the data voltage is Vdata1, and the value of the common voltage is Vcom1. The direction of the electric field of the pixel unit in frame n+1 is the same as that in frame n−1. That is, frame n−1 and frame n define a minimum period. The value and the direction of the electric field of the pixel unit in the following frames periodically repeat.
In each two adjacent frames, the two polarities of the data voltage are opposite, and the two polarities of the common voltage are also opposite accordingly. However, the absolute value of the common voltage is the same, and the absolute value of the voltage difference between the common voltage and the data voltage is the same. Therefore with each successive frame, the direction of the electric field of each pixel unit alternately changes, but the absolute value of the electric field of the pixel unit remains constant. The rotating angles of the liquid crystal molecules of each pixel unit are determined only by the absolute value of the electric field of the pixel unit. That is, when the absolute value of the electric field of the pixel unit is constant, the rotating angles of the liquid crystal molecules of the pixel unit are also constant.
Typically, the liquid crystal layer 14 is not pure. For example, a plurality of impurity ions (not shown) is mixed in the liquid crystal layer 14. The first and second alignment films 13 and 15 are made of organic materials, and capture the impurity ions easily. When the absolute value of the electric field of each pixel unit remains constant for a long time, the rotating angles of the liquid crystal molecules of the pixel unit are correspondingly constant. That is, the liquid crystal molecules have little effect on random motions of the impurity ions. Thus some of the impurity ions are captured by the first and second alignment films 13 and 15, and a residual direct current electric field (not shown) is generated between the first alignment film 13 and the second alignment film 15. Even if the direction of the electric field of the pixel unit changes, the residual direct current electric field may still subsist. The residual direct current electric field also drives the liquid crystal molecules to rotate. In any one frame, the residual direct current electric field may alter the rotating angle of each liquid crystal molecule. In addition, from frame to frame, the residual direct current electric field may cause each liquid crystal molecule to stay in the same position as the previous frame even when the liquid crystal molecule is being driven according to a video signal to change its rotating angle. Thus images of previous frames may continue to be viewed by a user. This problem is known as the residual image phenomenon.
What is needed, therefore, is an LCD device and a related driving method for the LCD device which can overcome the above-described deficiencies.